The present invention relates to a toner composition for electrophotography and more specifically to a toner composition for electrophotography which mainly comprises a urethane-modified polyester resin.
As the office automation techniques have advanced, demand for copying apparatuses and laser printers to which the electrophotographic method is applied has rapidly been increased and correspondingly the development of such apparatuses having higher quality have also been required. In particular, there has been a large requirement for the development of such apparatuses which make it possible to perform a fixing process at a low temperature from the viewpoint speeding up of copying processes, Energy-Savings and high safety. In addition, there has been a requirement for the development of a toner having excellent fixing ability at a low temperature.
To improve the fixing ability of a toner, there is a general need to reduce the viscosity of the toner in its molten state to enlarge the contact area between the toner and a substrate for fixing the toner. For this reason, there has been used, for instance, a toner-bonding resin having a low glass transition point or a low molecular weight in conventional techniques. However, these methods suffer from a variety of drawbacks such as these detailed below.
The toners must stably maintain their powdery form during applications and/or storage and, in other words, must be excellent in antiblocking properties. However, resins having a low glass transition point are in general insufficient in antiblocking properties.
Moreover, if these toners are used in the heat roll-fixing method, the toner transferred to hot rolls is liable to contaminate the transfer paper subsequently fed to a copying apparatus because of the direct contact between the hot rolls and the molten toner (i.e., it is liable to cause so-called offset phenomenon). This tendency becomes in general conspicuous when the molecular weight of the resin used is low.
Recently, an auto document feeder (ADF) for rapid copying has been connected to copying apparatuses, but portions of the toners adhered to the copying paper peels off from the copying paper and contaminates the copying paper subsequently supplied to the apparatus (so-called ADF contamination) if the adhesion of the toner to the paper is insufficient and/or if the strength of the toner is low. Resistance to ADF contamination is very important when a great deal of double copy operations are performed at one time and thus there has been a large demand for the improvement in resistance to ADF contamination.
As means for solving such problems, there have been proposed a variety of techniques, for instance, a toner which comprises a vinyl polymer properly crosslinked through the use of a crosslinking agent and a molecular weight modifier (see Japanese Patent Publication for Opposition Purpose (hereinafter referred to as "J. P. KOKOKU") No. Sho 51-2334); a toner which comprises structural units derived from an .alpha. ,.beta. -unsaturated ethylene monomer and whose molecular weight distribution is widened so that it has a ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn), Mw/Mn, ranging from 3.5 to 40 (see J. P. KOKOKU No. Sho 55-6805); and toners which comprise polymers blended so that the glass transition point, the molecular weight, the gel content or the like are properly controlled. Although the temperature range within which the toners can be fixed, in other words, the temperature range between the offset-generation temperature and the lower limit of fixing temperature, can be expanded or improved compared to that for conventional toners which comprise a single non-crosslinked resin having a relatively narrow molecular weight distribution, such a solution is still insufficient.
There has also been known a method for improving resistance to offset by partially crosslinkinq a polyester resin. In addition, the inventors of this invention have already proposed a method for improving resistance to offset which comprises blending a polyester resin having a low molecular weight and that having a high molecular weight in order to broaden the molecular weight distribution of the resulting polymer blend (see W087/04811).
However, while the method which comprises partially crosslinking a polyester resin makes it possible to prevent the offset phenomenon at a high temperature to some extent, the lower limit of fixing temperature is simultaneously increased. Therefore, it becomes difficult to perform fixing at a low temperature and the method cannot fulfill the requirements of speeding up of the copying operation and of energy-saving. Moreover, the partial crosslinkinq of a polyester resin impairs the dispersibility of toner additives in the polyester resin and the wettability with respect to transfer paper peculiar to the polyester resin. Thus, in practice, the method hardly improves the resistance to offset.
In addition, it has been concluded that if the method disclosed in WO87/04811 is adopted, the molecular weight distribution Of the resulting polymer blend becomes narrower than the predetermined one and that the desired effect cannot be necessarily achieved.